In radio equipment, such as a base station for cell phones, a filter circuit in which a resonator is connected in multiple stages is utilized. As this resonator, a resonator is utilized in which a columnar or a cylindrical dielectric resonator is housed in a metal case. However, there is a problem that such resonator has large volume. Meanwhile small dielectric resonators, each utilizing a dielectric substrate having a dielectric layer, are disclosed in Patent Literatures 1 and 2.
Patent Literature 1 discloses the dielectric resonator in which a pair of facing electrodes is formed on both main surfaces of the dielectric substrate, a plurality of through holes are provided between edges of the electrodes, and in which the electrodes are connected to each other through the through holes.
In addition, Patent Literature 2 discloses the resonator including the dielectric substrate and electrodes provided at both surfaces of the dielectric substrate, in which at least one of the electrodes of the surfaces is formed as a circular electrode. In Patent Literature 2, in the resonator, a plurality of through holes is provided in a penetrating manner along a periphery of the circular electrode in the dielectric substrate, an inside of the each through hole is set as an electrode non-forming portion in which the electrode is omitted, and open ends for enhancing electromagnetic field confinement are provided at the periphery of the circular electrode using the plurality of through holes. As a result of this, improvement in a Q value is achieved in the resonator described in Patent Literature 2.